6 THE HORSE FAMILY. 



Eocene Mammal not larger than a Fox, in which the lateral digits of 

 both feet are relatively as large as in the Tapir, while all the bones 

 of the feet are proportionately shorter than in the Horse, and all 

 three joints of each toe probably touched the ground. Species 

 intermediate between Mesohippus and Hijracotherium have been 

 named Flaffiolophus and Orohippus. Farther it is not at present 

 possible to carry the ancestry of the Horse, but there is little doubt 

 that Hyracotherium is descended from a still earlier five-toed 

 Mammal with a simpler type of cheek-teeth, and much shorter 

 foot-bones. This hypothetical animal doubtless walked on the whole 

 sole of its foot (plantigrade progression) instead of on the tips of 

 the toes, and was probably nearly related to the creature known 

 as Phenacodus, a cast of the skeleton of which is exhibited in 

 the Gallery of Fossil Mammals. For further details concerning 

 the extinct allies of the Horse see 'A Guide to the Fossil Mammals 

 and Birds in the Department of Geology and Palaeontology \ 

 rt ., - . The superficial (Pleistocene) deposits of South 



South American , . ^ ■ ^^ ^ ^ .^ 



_ ,. , „ America — more especially those oi the province or 



Extmct Horses. r> a i, n i ^- ^ 



Duenos Aires — have yielded remains oi two very 

 remarkable equine animals, Hippidium neogaum and Onohippidium 

 munizi. Of the former the model* of a nearly entire skeleton 

 (N.H. 3, fig. 3) is exhibited, while the latter is represented by a 

 cast of the skull (N.H. 17). In both genera the cheek-teeth 

 (as mentioned later) have shorter crowns and differ in several 

 details of structure from those of modern Horses. As mounted, 

 the skeleton stands 4 ft. 1 in., or 12^ hands, at the withers, while 

 the skull measures 23^ in. in total length. In an average 

 European horse-skeleton, standing 4 ft. 9J in., or 14 hands Ij in. 

 at the shoulder, the skull-length is about 23} in., or practically 

 the same as in the much smaller Hippidium. Although these 

 measurements suffice to show how disproportionately large is the 

 skull of the Hippidium, they by no means indicate the chief 

 peculiarities of that animal. Com.parison of the skull of the 

 former with that of an ordinary Horse shows a most remarkable 

 difference in the structure of the nasal region of the two species. 

 In me ordinary Horse the nasal bones are separated from the 

 maxillae, or upper jaw-bones, of either side by a slit of only some 



* The original of this model has been made tlie type of a second species, 

 but on very slight grounds. 



